Cell Structure Control And Functional Property Optimizing By Variable Pressure Foaming

Aluminum foam materials are a combination of various characteristics of metal materials and porous materials,which have a high specific strength,high specific stiffness,excellent mechanical properties,but also have many functional properties,such as deadening the noise reduction,damping vibration,electromagnetic shielding,energy absorption and shock protection,which can be used for the manufacture of precision machinery and high speed vehicles,train collision protection and other civilian areas,in the protective ability of military equipment and the anti-jamming ability of ascension has important significance.The properties of metal foam depends on the cell structure,such as cell size distribution,cell wall thickness,pore edge size and so on.Due to the existence of the critical rupture thickness of the cell wall,the inversely-proportional of the cell size and the relative density can be found,which limits the control of cell structure and the performance optimization of the aluminum foam materials.For the realization of active control of cell structure,the influence of environmental pressure on liquid metal foam expansion process and pore coalescence process and the critical rupture thickness were investigated,developing a new preparation method basing on variable pressure foaming.The structure characterization and performance testing of the materials were carried out,and the effect of cell size on the mechanical properties of foamed aluminum was studied.Main contents include:Using laser range-finding technology,the effects of foaming pressure on expansion process are studied,measurement of the height and time curve of expansion process under different pressure.The expansion process of the foam is suppressed under increased pressure.As the pressure increases,the decomposition rate of the foaming agent is slowed down,the maximum expansion rate decreases,and the maximum expansion time is pushed.Under the positive pressure,the cell wall stability is enhanced,and the gas content of foam surface layer because of cell rupture is reduced,and the actual expansion rate of the foam body is higher than that based on the ideal gas law.Cell structure characterization results show that the average cell diameter of foam becomes smaller with the increase of pressure,from 3.1mm at atmospheric pressure to 0.8mm at 0.5MPa,and the cell size changes with the pressure non-linearly.Microscopy experiments on the cell wall thickness measurements show that the cell wall thickness by 57?m reduced to 21 ?m under increased pressure,which indicates that the cell wall has better stability under high pressure and pore coalescence is inhibited.Moreover,because of the decrease of the thickness of the cell wall,the inversely-proportional relationship between the average cell size and the relative density is broken.Quasi static compression tests were carried out on the samples prepared under different pressures.The results show that the stress fluctuation of the platform is gradually reduced and the coincidence of the sample curve with the same density becomes better when the pore size is refined and uniform.The relationship between density and compressive strength shows that the compressive properties of the foam aluminum is linear with the relative density,and the dispersion become small.